The present invention relates to a control circuit for a stepping motor for positioning a transducer used in a magnetic disk unit or an optical disk unit, and more particularly to a control circuit for a stepping motor which enables positioning of a transducer in a short time.
A stepping motor has been widely used in a magnetic disk unit or an optical disk unit as a relatively inexpensive positioning mechanism. As drive method for the stepping motor, a constant velocity control method and a pedestal type velocity control method have been known by such as Japanese Patent Application Laid-Open No. 55-136896. Those drive methods are explained with reference to FIG. 1.
In the constant velocity control method, a control pulse signal having a substantially constant pulse interval T.sub.C is supplied to the control circuit for the stepping motor from a high level controller so that a stepping pulse P.sub.C shown in FIG. 1(a) is supplied to the stepping motor. As a result, the stepping motor is rotated at a substantially constant speed W.sub.C from time t.sub.o to time t.sub.e as shown by a line a' in FIG. 1. At time t.sub.s, a transducer driven by the stepping motor is positioned at a desired position. If the interval T.sub.C of the stepping pulse is sufficiently large, the stepping motor is intermittently driven.
In the pedestal type velocity control method, a plurality of control pulse signals are successively supplied to the stepping motor control circuit from the higher level controller during time period T.sub.B. The stepping motor control circuit accelerates the stepping motor in accordance with the number of applied control pulse signals from time t.sub.1 to time t.sub.2 as shown by b' in FIG. 1, rotates the stepping motor at a maximum speed W.sub.T from time t.sub.2 to time t.sub.3 and decelerates the stepping motor after time t.sub.3. Since the rotating speed W.sub.T is sufficiently higher than the rotating speed W.sub.C, the transducer driven by the stepping motor is positioned to the desired position at time t.sub.4 which is earlier than time t.sub.s. A data for producing the stepping pulses to be supplied to the stepping motor to perform the pedestal type velocity control is usually stored in a read-only memory (ROM).
In the prior art control circuit for the stepping motor, one of the above two velocity control methods has been used.
Referring to FIG. 2, the prior art control method for the stepping motor is explained. In a step S.sub.1, a count of a pulse counter of the control circuit is set to an initial count N=1. In a step S.sub.2, the control circuit checks if the control pulse has been supplied. If it has not been supplied, the step S.sub.2 is again repeated. If the control pulse signal is supplied to the control circuit, a time interval T.sub.I between the applied control pulse and the next input pulse is measured in a step S.sub.3. If the time interval T.sub.I is T.sub.I .gtoreq.X (where X is a predetermined time period), that is, if the next control pulse signal is not applied within the time period X, the stepping motor is driven by one step in a step S.sub.4. In a step S.sub.5, the count N of the counter is set to N-1. In a step S.sub.6, the count N of the counter is checked. If N=0, the process returns to the step S.sub.1. Through the steps S.sub.1, S.sub.2, S.sub.3, S.sub.4, S.sub.5 and S.sub.6, the constant velocity control method is performed. By repeating the steps S.sub.1 to S.sub.6, the constant velocity control is attained. In the step S.sub.3, if the time interval T.sub.I of the input control pulse signal is T.sub.I &lt;X, the count N of the counter is set to N+1 in a step S.sub.7. While the control pulse signals having the time interval T.sub.I (T.sub.I &lt;X) are applied to the steps S.sub.2, S.sub.3 and S.sub.7, the count N of the counter is incremented by "1" each time. In the step S.sub.3, if the next pulse is not supplied within the time period X, the stepping motor is driven by one step in the step S.sub.4. In the step S.sub.5, the count N of the counter is decremented by one, and in the step S.sub.6, the count N of the counter is checked. If N.noteq.0, stepping pulses for the pedestal type velocity control are generated in steps S.sub.8 and S.sub.9. In the step S.sub.8, a time interval T(N) of the stepping pulses to be supplied to the stepping motor is read out of the read-only memory (ROM) in accordance with the count N of the counter. In the step S.sub.9, the stepping pulses having the pulse interval T(N) are produced based on the data read in the step S.sub.8. Then, the process returns to the step S.sub.4 in which the stepping pulses are supplied to the stepping motor. The steps S.sub.4, S.sub.5, S.sub.6, S.sub.8 and S.sub.9 are repeated until the count N of the counter reaches zero. The pedestal type velocity control is attained by the steps S.sub.1, S.sub.2, S.sub.3, S.sub.7, . . . , S.sub.2, S.sub.3, S.sub.7, . . . , S.sub.4, S.sub.5, S.sub.6, S.sub.8, S.sub.9, . . . , S.sub.4, S.sub.5, S.sub.6, S.sub.8, S.sub.9, . . . , S.sub.4, S.sub.5, S.sub.6. The time interval T(N) of the stepping pulses is a function of N. When the count N is larger than a first predetermined value P.sub.1, the pulse interval T(N) gradually decreases, when the count N is between the first value P.sub.1 and a second value P.sub.2, the pulse interval T(N) is constant, and when the count N is smaller than the second value P.sub.2, the pulse interval T(N) gradually increases. By the change of the pulse interval T(N), the pedestal type velocity control is attained.
As described above in the prior art pedestal type velocity control method for the stepping motor, the drive of the stepping motor is started after the reception of all control pulse signals sent from the higher level controller to the control circuit. Therefore, a long time is required to position the transducer. This time increases as the number of control pulse signals or the number of stepping pulses increases.